Process of making aluminum coated steel of high reflectivity



3 069 769 PRGCESS: 8F MAKIfs G ALUMINUM COATED STEEL UK HIGHREFLECTIVITY Noel W. Parks, Middletown, Ohio, assignor to Armco gtgelCorporation, Middletown, ()hio, a corporafion of 10 No Brawing. FiledJuly 28, 1958, Ser. No. 751,157 4 Claims. (Cl. 29528) The invention hasto do with the problem of making iron or steel sheet stock which iscoated with aluminum by a process involving the wetting of the basestock surface with molten aluminum, in such a way that the aluminumsurface of the stock has great smoothness and high reflectivity. Thesequalities enable the stock to be employed to better advantage in manyuses including both decorative uses, uses in the fabrication of articleswhere an aluminum surface is desired, insulatiye uses, and uses for thereflection and concentration of heat or infrared radiations, as, forexample, in electric wall heaters.

Methods and apparatus for coating iron or steel sheet stock (usuallystrip stock of sheet width and gauge) with aluminum have hitherto beendeveloped. The present invention is not limited to any specific coatingprocess, although the characteristics of the coated stock should bethose hereinafter set forth. By way of example the Sendzimir Patent No.2,110,893, dated March 15, 1938, and the Oganowski Patent No. 2,437,919,dated Mar-ch 16, 1948, may be referred to as teaching suitable methodsand apparatus for coating.

When iron or steel stock is coated by these and other atent methods arelatively bright surface is produced; but the surface is notmirror-smooth nor as reflective as, for example, sheet aluminum whichhas been rolled to gauge in a carefully adjusted mill with smooth rolls,the finishing pass or passes being made by highly polished or burnishedrolls. Moreover, the surface is generally characterized by inequalities,principally heavier and lighter areas.

It is an object of the invention to provide a way of improving the re.ectivity, smoothness and coating perfection of iron or steel stock hotcoated with aluminum.

It is an object of the invention to provide a process of this characterwhich is inexpensive and does not add significantly to the cost of theproduct.

It is an object of the invention to provide a process of the characteroutlined which does not operate to change the physical characteristicsof the product.

These and other objects of the invention which will be set forthhereinafter or will be apparent to one skilled in the art upon readingthese specifications, are accomplished by that procedure and in thatproduct of which certain exemplary embodiments will now be described.

The products of this invention are normally coated with aluminum in atleast commercially pure form; but it is not intended to exclude from theclaims which follow, iron and steel stocks (or other base metal stocks)coated with alloys consisting preponderantly of aluminum but containingother alloying elements such as silicon, magnesium, and the like inminor quantity.

Suitable coated products for the practice of this invention may havecoatings of aluminum or aluminum alloy of about one ounce per squarefoot. In the coating of iron or steel base stock with aluminum by themethods referred to above, there is normally formed between the basestock and the aluminum coating a thin layer of iron-aluminum alloy. Suchan alloy layer can be toereted in the present process; but the coatingshould be carried on in such a way that there is a definite top layer ofaluminum or aluminum alloy which is pure in the sense that it is notsubstantially alloyed with the metal of the base stock. In a preferableproduct the overlying ice layer of aluminum, as distinguished from theinterface alloy, should have a thickness of about .005 to about .0005inch.

With stock of this character, it is impossible to obtain the bright andreflective surfaces contemplated by this invention through the expedientof rolling the product between bright or burnished rolls. The basestock, treated in accordance with the patents which have been set forthabove, is generally softened or annealed in the reducing furnace throughwhich it passes before it enters the molten bath of aluminum or aluminumalloy, and consequently in many instances can be given a certain amountof temper rolling, say up to an elongation of about 2%. But because ofthe roughnesses and inequalities in the surface, as described above, anattempt to roll the coated stocks between burnished or polished workingrolls with an elongation of about 2% or less will result in a streakedor mottled appearance which is unpleasant and makes for low overallreflectivity. This is due to the fact that at such elongations thepolished rolls are not able to contact the whole of the coated surfaceunder equal pressure. The mottling can be descreased by increasing theelongation; but in most instances even at 5% or more elongation it isquite impossible to make the whole surface uniform. Further, suchelongations, and indeed any elongations above about 2%, tend to hardenthe base stock and make it unfit for many fabricating operations. Thus,it has been hitherto impossible to treat aluminum coated iron or steelbase sheet stock by rolling with polished rolls so as to obtain auniformly bright surface of high reflectivity; and the art has had nosolution for this problem.

In accordance with the present invention, it has been discovered that itis possible to give to the aluminum or aluminum alloy coated base stocka preliminary treatment which changes the character of the light metalcoating and unifies and redistributes it, in such a way that asubsequent rolling with bright or burnished rolls will produce amirror-smooth and highly reflective surface which is uniform andcompletely free from the mottling effect described above. It has furtherbeen found that the preliminary and subsequent treatme ts can be carriedon without undue elongation of the base stock, and well within thelimits of normal desirable temper rolling.

Briefly, in the practice of the invention, the coated product is firstrolled through rolls having a uniform rough'ess as will hereinafter bedescribed. This creates a dull but uniform matte finish which has beenfound to roll flat and bright upon the subsequent treatment. Puttingthis another way, the coating after the first treatment may be describedas characterized by hills and valleys, in the formation of which thecoating has been redistributed and unified, with the elimination ofsurface defects. When such a treated coating is subsequently rolled withbright finished rolls, the tops of the hills are flattened down into amirror finish. In the successful practice of the invention, it isnecessary to produce hilllike formations of a suitable height anddistribution.

For the production of the dull but uniform matte finish, a mill isemployed with carefully and uniformly roughened rolls. These rol s arepreferably shot blasted or so-called Pangborn" rolls which have beentreated with a shot or grit ranging from about number to number asunderstood in the art of making shot blasted ro ls. A roll shot blastedwith number 120 shot or grit will have a surface roughness of 50 to 60micro inches R.M.S. (root mean square), as measured on a standardprofilometer. It has been found that with rolls uniformly treated withshot of the sizes set forth, the coating on the iron or steel base stockwill be uniformly converted to the desired matte finish in a temperrolling which does not produce more than about 1% elongation.

sheaves If rolls are employed treated with a shot or grit substantiallysmaller than number 150, a matte finish will be formed on only parts ofthe area of the stock surface, and the effect will be mottled. If therolls have been treated with a shot or grit substantially larger thannumher 120, it may be found diflicult to avoid a mottling of the surfaceat an elongtaion of about 1% or less; but in any event, the subsequenttreatment with the bright or buffed rolls Will not serve to flatten downthe hi.ls of the coating, and the resultant finish as observed byreflected light at a relatively high angle will be characterized by agrain which has an appearanec somewhat like the grain of moroccanleather. The reflectivity and brightness of the product will also beimpaired.

It will be understood by those skilled in the art that the roughness ofshot blasted or Panborn rolls tends to wear away in time, and careshould be taken to keep the rolls in good condition, refinishing them asmay be required. The need for refinishing can be ascertained byinspection of the rolls; but when the shot blasting indentations haveworn away to an appreciable degree, difficulty will be found inobtaining a dull matte finish which is uniform on the product. In otherwords, the appearance of mottling can be taken as an indication that newrolls should be substituted, or the old rolls retreated.

When a suitable dull matte finish has been produced as set forth above,the second step of the treatment, namely a temper rolling of the productusing highly polished or buffed Working rolls, will serve to produce asmooth bright finish with high reflectivity, devoid both of mottling,and of any pattern persisting from the roughness which was initiallyimparted to the coating. Any suitable type of buffed or polished rollsmay be employed for this purpose, a suitable type of roll being thatknown in the trade under the name of Buffed Rolls, and having typicallya profilometer reading of 1.5 micro inches R.M.S. it will be understoodthat such rolls also tend to wear, and therefore the rolis employedshould be new or of equivalent brightness. Wear tends to produce aroughness in the burnishing rolls, and if this occurs a finish ofinferior reflectivity will result. The invention is not limited to theuse of rolls having any particular degree of polish or burnishedcharacter, it being understood that the higher the polish of the rollsthe smoother the surface will be.

The production of a uniformly smooth bright finish, devoid of mottling,and possessing high reflectivity can be attained after the initialtreatment with an elongation of 1% or less. Thus the total elongationproduced in the two treatments can be held to a value of 2% or lesswhich is well within the normal permissible limits of temper rolling forthe product.

it may be stated that, given a normal perfection of coating, the thinnerthe layer of pure aluminum or aluminum alloy (as distinguished from theinterface layer of iron-aluminum alloy) the less total elongation willbe required in the successive treatments which have been described.

The process has been found to work consistently and to give consistentlygood results. It will be understood that the specific brightness andreflectivity of the finished product will depend to a certain extentupon the finish of the rolls used in the second step. The brightnes ofthe treated strip may be measured with a reflectance meter. By way of anexemplary but non-limiting illustration, an aluminum-coated iron orsteel strip, in the ascoated condition, showed a reflectivity of 16%.When this product was given a dull but uniform matte finish by beingrolled with number 120 shot Pangborn rolls with an elongation of 1%, itsreflectivity was found to be 19%. When the product thereafter was rolledwith about 1% elongation with Buffed Rolls, it exhibited a reflectivityin excess of 90%. Its surface was completely devoid of mottling, and wasmirror-smooth and bright. It was found that the coating had beenunified; and the surface was devoid of the appearance of inequalities.The product was excellently adapted for fabrication, having been temperrolled with an elongation of not over 2%. Its heat reflectivity wasfound to be equivalent to or better than bright rolled solid aluminum,buffed chrominum plated stainless steel, buffed copper and buffed 188stainless steel.

It will be understood by the skilled worker in the art that no surfacetreatment should be applied on the aluminum coating after formation asherein set forth, since such surface treatments tend to produce a fog yfilm after bright rolling. To the extent that protection for the surfaceis desired, the product may be covered With paper either by wrapping,interleaving, or with the use of a pressure-sensitive adhesive, untilthe product is used for fabrication, installation, or the like.

Modifications may be made in the invention without departing from thespirit of it. The invention having been described in an exemplaryembodiment, what is claimed as new and desired to be secured by LettersPatent is:

1. A process of producing a base metal sheet stock coated with aluminumor aluminum alloy and having a bright, uniform, mirror-like finish ofhigh reflectivity which comprises associating aluminum or aluminum alloywith a base metal sheet stock by hot coating to a thickness of .005 inchmaximum, rolling the coated sheet stock with an, elongation notexceeding about 1% between a pair of working rolls having a uniformlyrough surface characterized by shot blasting with grit ranging fromnumber 120 to number 150 whereby to produce a dull uniform matte finishon the product, and thereafter temper rolling the product with anelongation not greater than about 1% between a pair of highly polishedworking rolls having a profilometer reading of about 1.5 micro incheswhereby to smooth out the matte finish previously produced.

2. A process of producing an aluminum coated iron or steel strip stockof sheet gauge which comprises hot coating said strip stock withaluminum so as to produce a coating Weight not exceeding about one ounceper square foot, thereafter temper rolling said coated stock through apair of shot blasted working rolls treated with shot ranging fromsubstantially number 120 to substantially number 150, the elongation notexceeding about 1%, and finally temper rolling said strip with anelongation not exceeding substantially 1% between a pair of highlypolished working rolls having a profilometer reading of about 1.5 microinches.

3. The process claimed in claim 2 wherein the aluminum on the surface ofsaid stock, as distinguished from any aluminum-iron interface alloybetween said aluminum and said stock, has a thickness of at least about.0005 inch.

4. A process of producing an iron or steel strip stock of sheet widthand gauge having a uniform, mirror-like finish of high reflectivity,which comprises an iron or steel stock hot coated with a material chosenfrom a class consisting of aluminum and aluminum alloys in whichaluminum preponderates, the thickness of the said coating being betweenabout .0005 and .005 inch, and in a series of rolling steps the totalelongation of which does not exceed about 2%, first redistributing saidcoating by forming uniform proturberances and depressions therein byrolling between a pair of working rolls having a profilomw eter readingof about to micro inches to provide a dull matte finish of uniformcharacter throughout the coated area, and then reducing saidproturberances and depressions to a uniformly smooth and bright surfaceby rolling with highly polished rolls.

References Cited in the file of this patent UNITED STATES PATENTS1,695,910 Biggert Dec. 18, 1928 (Other references on following page)UNITED STATES PATENTS 2,773,302 Wa ts0n Dec. 11, 1956 2,084,327 EdwardsJune 22, 1937 29287 51111111 1960 2,126,578 Roemer Aug. 9, 1938 OTHERREFERENCES 214801711 Canon 30, 1949 5 Aluminum and Its Application, byHiram Brown, 2,490,543 Robertson 6, 1949 Pitman Publishing C0,, NewYork, copyright 1948; pages 2,539,247 Hensel Jan. 23, 1951 109-112,122-124. 2,687,565 Schaefer Aug. 31, 1954 Handbook of Aluminum,published by Aluminum 2,708,304 Lundin May 17, 1955 Company of Canada,copyright 1957; pages 2633, 130- 2,752,3os Turner June 26, 1956

1. A PROCESS OF PRODUCING A BASE METAL SHEET STOCK COATED WITH ALUMINUMOR ALUMINUM ALLOY AND HAVING A BRIGHT, UNIFORM, MIRROR-LIKE FINISH OFHIGH REFLECTIVITY WHICH COMPRISES ASSOCIATING ALUMINUM OR ALUMINUM ALLOYWITH A BASE METAL SHEET STOCK BY HOT COATING TO A THICK NESS OF .005INCH MAXIMUM, ROLLING THE COATED SHEET STOCK WITH AN ELONGATION NOTEXCEEDING ABOUT 1% BETWEEN A PAIR OF WORKING ROLLS HAVING A UNIFORMLYROUGH SURFACE CHARACTERIZED BY SHOT BLASTING WITH GRIT RANGING FROMNUMBER 120 TO NUMBER 150 WHEREBY TO PRODUCE A DULL UNIFORM MATTE FINISHON THE PRODUCT, AND THEREAFTER TEMPER ROLLING THE PRODUCT WITH ANELONGATION NOT GREATER THAN ABOUT 1% BETWEEN A PAIR OF HIGHLY POLISHEDWORKING ROLLS HAVING A PROFILOMETER READING OF ABOUT 1.5 MICRO INCHESWHEREBY TO SMOOTH OUT THE MATTE FINISH PREVIOUSLY PRODUCED.